We have used electron paramagnetic resonance (EPR) spectroscopy for investigating the properties of spins, such as those carried by polarons which carry both spin and charge in poly (meta/para phenylene) PMPP: CdS doped Mn based nanocomposites. To identify the nature of paramagnetic species in PMPP matrix, we have studied the effect of different physical parameters. It was found that we are in presence of trapped polarons and localized spins which concentration has been estimated. Moreover, spin–spin and spin–lattice relaxation rates have been calculated. Then, we discussed the results of optical and EPR study on the hybrid nanocomposite (CdS nanostructures, doped with manganese (II) ions, incorporated in PMPP conjugated polymer matrix). The optical spectra of these nanocomposites were compared to the existing models of energy levels in quantum dots. Moreover, by the use of electronic paramagnetic resonance, conclusions about the location and the symmetry of Mn2+ ions have been drawn. The nanocomposite energy gap is in the 3.2–3.3 eV range. The size of the nanoparticle is about 3.3 nm and Mn2+ ions are located at or near the nanoparticle surface.

We have investigated the doping mechanism of pentacene with iodine and its impact on the structure and on the electronic properties of single crystals, powders, and thin films in a large range of iodine concentration up to six iodine per pentacene (PEN) molecule (I/PEN = 6). Three regimes of doping have been identified. In the low doping regime I/PEN < 0.05, the pristine pentacene structure of single crystals is maintained. Electron spin resonance (ESR) evidences a Pauli susceptibility, that is, the characteristic fingerprint of delocalized holes in the valence band of pentacene. In the intermediate doping regime (0.1 < I/PEN less than or equal to 2.0), iodine diffuses between the (a,b) planes of the pentacene structure and forms an intercalate. Charge transfer between iodine and pentacene is witnessed by both UV-vis and IR signatures of PEN+ cations and related species, for example, cation dimers (PEN+)(2) and typical Raman signatures of the I-3(-) and I-5(-) species. Spin pairing of pentacene cation radicals is further supported by the observation of a thermally activated behavior of the ESR spin susceptibility. In the heavy doping regime (2 < I/PEN less than or equal to 6), all traces of structural order vanish, indicating that iodine penetrates within the (a,b) planes of the intercalate in a disordered manner, forming an amorphous-like material. This high degree of disorder results in increased charge localization. Most spin/charge species are ESR-silent and only a limited fraction (a few percents) exhibits a Curie-like susceptibility. Because of disorder, the macroscopic conductivity of doped pentacene single crystals does not exceed a few S/cm at 300 K.

In a molecular semiconductor, a charged molecule experiences a lattice relaxation which reorganizes it into a cation or an anion-radical. This species is not, in general, a polaron. By using calculations of the geometry and the electronic structure both ab initio and at the semi-empirical levels, we have explored the conditions of polaron formation in a molecular model system: oligophenylene–vinylenes, PVs of increasing sizes. The symmetry breaking occurs at sizes larger than five monomers for the anion and six monomers for the cation. The driving force for this process is primarily the charge–lattice coupling through the stretching mode at 1600 cm−1.

The magnetic properties of a series of stable diradicals based on imino nitroxide (IN) and nitronyl nitroxide (NN) are investigated. Both radical fragments are coupled through various diamagnetic conjugated organic spacers made of aromatic rings (4) alternating with either a triple (1x, 3) or a double bond (2x), where x indicates the length of the molecules with respect to the repeating unit. The optimised geometry and the corresponding electronic structure of the whole series (x=0,…,10) have been determined by means of semiempirical calculations based on the NDDO approximation within the AM1 parametrisation scheme. The singlet–triplet splitting has then been studied using configuration interaction within different active spaces. It turns out that the most important parameters are: (a) of geometrical nature (e.g. twist angles between the radical moieties and the planar conjugated spacer); and (b) of topological nature (e.g. the radical substitution at ortho, meta, and para positions of phenyl cycles). The results of the calculations enable us to predict some rules in relation to the experimental observations: (a) for a given value of x, the hierarchy of the exchange coupling within series of derivatives is such as J(R1=R2=NN)>J(R1=IN, R2=NN)>J(R1=R2=IN); (b) whatever the nature of the radical substituent, the singlet is the ground state and the exchange coupling decreases nearly exponentially with the length of the spacer, i.e. as x increases. The exchange coupling is still efficient up to x=8 for R1=R2=IN (experimentally it has been previously found efficient up to x=5 in the para position).

Unidirectionally oriented films of x-form lithium phthalocyanine (PcLi) have been obtained by electrodeposition onto ITO substrate and their magnetic properties studied by Electron Spin Resonance (ESR). The tuning of the microcrystallite mean size and orientation is obtained by the control of the deposition parameters including electrolysis time and the chemical nature of the solvent. The magnetic properties of films of thickness ca. 1 mum are found to be identical to those of bulk single crystals i.e. they show a narrow ESR signal of linewidth ca. 20-30 mG under vaccum which broadens linearly with the dioxygen pressure. Our ESR study also reveals some exceptional mesoscopic effects. The ESR signal linewidth DeltaH(ESR) is found to follow a power law of the mean microcrystallite size.

The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu2+ ions, have been studied by means of electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu2+ ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, Tg, have also been measured for these borate glasses and it is found that the Tg decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, Λth, of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP.

The magnetic properties of lithium phthalocyanine thin films obtained by electrodeposition onto ITO glass substrates are investigated via electron paramagnetic resonance (EPR). These properties:are shown to depend on several parameters such as (i) the electrolysis time (t(e)), which controls the size of the crystallites as well as their unidirectional ordering in the thin films and (ii) the nature of the solvent used during electrodeposition. Highly oriented x-form thin films of thickness ca. 2 mu m (t(e) approximate to 30 min) exhibit similar properties as observed far single crystals: (i) a narrow EPR signal with a linewidth in the range 20-30 mG which broadens linearly with the oxygen pressure and (ii) a low dimensional behaviour of the linewidth anisotropy. A correlation is found between the mean size of the microcrystallites in the thin films and the EPR signal linewidth according to the equation Delta H-pp = K[I](alpha), where K is a constant and alpha approximate to 0.38 +/- 0.04. The temperature dependence of the EPR signal linewidth and the magnetic susceptibility are analyzed in terms of spin S=1/2 solitons which are thermally generated with a typical activation energy E-a(chi) of ca. 0.04-0.06 eV.

The difference in ability of a quartet molecule to interact with the neighboring solvent molecules is influenced by the molecular surface characteristics of the two diastereomers of the molecule. Therefore, molecular parameters like the surface area and the fractal dimension (see figure) control some of their physicochemical properties, such as their differential chromatographic retention of these two diastereomers, their isomerization equilibrium, and tumbling processes in solid or viscous amorphous matrices.

Electronic paramagnetic resonance (EPR) spectra of xLi2O·(30 − x)MoO3·70B2O3 glasses (0 ≤ x ≤ 25) containing 2 mol% V2O5 were studied between room and liquid-helium temperatures at X-band frequencies. The spectra are typical of V4+ ions present in vanadyl (VO2+) form in the glass. The spin-Hamiltonian parameters (SHP), g∥ g⊥, A∥, and A⊥, the dipolar hyperfine coupling parameter, P, and the Fermi constant interaction parameter, k, were evaluated. It was found that these parameters are independent of the temperature. Results suggest that the tetragonal distortion of the V4+O6 complex increases with increase in the Li2O:MoO3 ratio. It was also found that the increase in this ratio results in an expansion of the 3dxy orbit of unpaired electrons in the vanadium ion and that the spin-Hamiltonian parameters do not depend on the value of theoretical optical basicity.

Results are reported for electron paramagnetic resonance (EPR) and optical absorption structural investigations of Mn2+ ions in alkali barium borophosphate glasses at room temperature. The Mn2+ EPR hyperfine sextet centered at g = 2 was observed in all the glasses for 1 mol% Mn2O3. The absorption spectra of Mn2+ ions in these glasses showed broad absorption bands characteristic of Mn2+ ions in octahedral symmetry. From the EPR and optical results, it is concluded that the site symmetry around Mn2+ ions is octahedral, and the nature of the bonding character is dominantly ionic.

The results of measurement of electron paramagnetic resonance and optical absorption spectra at room temperature of alkali barium chlorophosphate glasses (BaCl2–M2O–P2O5) doped with 1 mol% CuO are presented for various alkali compositions (M=Li, Na, K, Li–Na, Na–K and K–Li). The results indicate that the mixed alkalies do not produce alterations of the glass network. The ground state of Cu2+ is the d〈x2−y2〉 orbital and the site symmetry that exists around the Cu2+ ions is octahedral with a tetragonal elongation. The theoretical optical basicity of the glasses have been evaluated and the changes in optical basicity are independent of changes in spin-Hamiltonian parameters of the Cu2+. The optical absorption spectra of Cu2+ ions for all the glass samples was single asymmetric band which corresponds to a 2B1g→2B2g transition. By correlating the spectral data, the molecular orbital coefficients α2 and β12 for Cu2+ have been evaluated and results indicate that there is covalency for the in-plane σ-bonding and that the in-plane π-bonding is significantly ionic in nature.

This study focuses on the correlation existing between the structure and the continuous dark conductivity in single crystals, compacted powders and thin films of the lithium phthalocyanine radical (PcLi). The dc conductivity at room temperature measured in PcLi compacted powders is of about 2-4 . 10(-5) (Ohm cm)(-1) for the x structure and 1-2 . 10(-4) (Ohm cm)(-1) for the alpha structure. The temperature dependence of the conductivity measured in the range 130-300 K hints at an activated-like behaviour with activation energies in the range 0.1-0.2 eV depending on the structure. PcLi thin films grow either in the x or the alpha structure depending on the substrate temperature (T-s) during deposition on the glass substrate. The conductivity shows a clear and strong dependence on the substrate temperature T-s and increases between 1 . 10(-6) (Ohm cm)(-1) for T-s = 25 degrees C and 5 . 10(-4) (Ohm cm)(-1) for T-s = 200 degrees C. Annealing of the PcLi thin films obtained for T-s = 25 degrees C at 200 degrees C under vacuum results in a clear increase of the conductivity by more than one order of magnitude. The conductivity is independent on the presence of oxygen in powdered samples and single crystals, indicating the intrinsic nature of the semiconducting behaviour. Contrary to alpha-PcLi thin films (T-s similar to 200 degrees C), the conductivity in PcLi x-form thin films (25 degrees C < T-s < 125 degrees C) is sensitive to the presence of ambient atmosphere (sigma(air)/sigma(vac) similar to 2.3 at 300 K for T-s = 24 degrees C). This weak variation of the conductivity cannot be attributed to extrinsic charge carrier generation as observed for other metallophthalocyanines. (C) 1998 Elsevier Science S.A. All rights reserved.

Mn2+-doped nickel bis (hydrogen maleate) hexahydrate single crystals have been studied by electron paramagnetic resonance (EPR) from room temperature to liquid nitrogen temperature at X-band. From the observed EPR spectrum the spin-Hamiltonian parameters have been evaluated and the Mn2+ was found to substitute for Ni2+ ions in the host lattice. The observation of unusual well resolved Mn2+ spectrum at room temperature and its broadening on lowering the temperature in Ni[C4H3O4]2·6H2O: Mn2+ single crystals have been explained in terms of the `spin-quenching concept'. The observed shift in the g-factor of the Mn2+ ion from that in diamagnetic hosts is probably due to the Ni2+ ions, which produce a local static magnetic field at the site of Mn2+ ions. The covalency percentage of the Mn–O bond has been estimated to be 9.2%.

The magnetic properties of the lithium phthalocyanine radical are investigated for the x,α and β crystal structures via electron paramagnetic resonance (EPR). Different magnetic behaviours (susceptibility, line shape) are evidenced in PcLi powders and interpreted in terms of the molecular packing. A Curie–Weiss susceptibility is observed for the α-phase with a mean-field temperature, TCW of ca. 4.5±1 K, whereas a two-component susceptibility must be considered for the x-phase (powders grown in acetonitrile) involving a Curie–Weiss contribution (TCWca. –3±1 K) and a thermally activated contribution with an activation energy Ea of ca. 0.04 eV. For the PcLi β structure two temperature domains can be distinguished: for T>50 K a Curie–Weiss susceptibility with a mean-field temperatureTCW of ca. –60±5 K is observed while for T<20 K a Curie–Weiss-like susceptibility withTCWca. –7±1 K dominates. The overall magnetic properties of the PcLi polymorphs are discussed in terms of McConnell’s mechanism for ferromagnetism and antiferromagnetism in solid free radicals. The magnetic properties of β-PcLi needles (magnetic susceptibility, anisotropy of spin diffusion) have been investigated. Adsorbed oxygen is shown to alter strongly the line shape, the linewidth and the g-factor anisotropy in β-PcLi crystallites and powders at temperatures below 55 K.

Lithium phthalocyanine (PcLi) is an intrinsic molecular semiconductor. This radical can crystallize in various crystalline phases, x, α, β, which show different properties (conductivity, magnetism, oxygen sensitivity). The x and β polymorphs may be studied on single crystals. Thin films are obtained by sublimation under vacuum on glass or oriented substrates. These films present different types of organization and crystalline structures depending on the nature of the substrate, its temperature, subsequent heating and solvent treatments, etc. The only x-form shows the following properties: (i) the presence of channels in which the dioxygen molecules can migrate; (ii) a strong overlap between consecutive PcLi molecules in a stack and a very efficient spin diffusion revealed by a very narrow EPR line. The latter is strongly broadened in the presence of oxygen. This chemically stable system is then a very good candidate for magnetometry and oximetry applications, in particular in aggressive environments. Nevertheless, these exceptional properties are not yet observed on x-form thin films.

It has been long thought that microscopic mechanisms related to magnetic short-range order were responsible for the temperature dependence of the electron paramagnetic resonance g tensor in low-dimensional magnetic systems. We show here that the demagnetizing field can explain qualitatively and quantitatively the observed features, i.e., (i) the g shift, variation of the g value, (ii) the presence of magic angles where there is no g shift, and (iii) the reorientation of the g tensor with temperature. These features are discussed theoretically and supported experimentally in purely organic insulating compounds. Previous results obtained on two different nitroxide derivatives art revisited in this framework. The role of the demagnetizing field may probably be generalized to most low-dimensional molecular materials.

Thin films of lithium phthalocyanine (PcLi) were prepared by vacuum deposition on a glass substrate at various substrate temperatures. Modifications of the electron spin resonance signal (shape, position) with the substrate temperature are evidenced and explained by means of the x-like → α phase transformation. Moreover, thin films deposited on glass substrate show a strong preferential ordering of the molecules forming columns which are lying in the plane of the substrate, the resulting orientation of the molecules being almost perpendicular to the substrate plane. This preferential ordering results in an anisotropic spin transport depending upon the substrate temperature. It is demonstrated that the demagnetizing field can account for the temperature dependence and the angular variation of the g-factor in PcLi thin films.

The properties of a series of stable diradicals based either on iminonitroxide (IN) or nitronylnitroxide (NN) are described. Both radical fragments are coupled by various phenylene and phenylethynylene (triple bond) bridges. They are characterized by EPR studies both in solution and in glassy state. Magnetic coupling is evidenced at a very large distance between both radical fragments (up to 36 Angstrom). The intramolecular coupling is found to be antiferromagnetic in all cases and in particular for meta- or ortho-substituted phenylene. Semiempirical calculations using configuration interaction show that the linkers with triple bonds induce a stronger conjugated character than the corresponding ones involving a double bond, in agreement with the experimental optical spectra, and that the relevant geometrical parameters are the twist angles; the corresponding barriers of rotation are then evaluated. Calculations show that, for some values of the twist angle, an antiferromagnetic state may be stabilized.

The effect of acetone vapours on the phase transformation x → α was studied by means of electron microscopy, X-ray diffractometry, electron spin resonance (ESR) and optical spectroscopy on thin films and powders of lithium phthalocyanine (PcLi). The exposure of amorphous films of PcLi, deposited at a substrate temperature of 15 °C onto glass slides, to solvent vapours of acetone at room temperature, induces important changes in the morphology and crystalline structure of the films. Exposure to acetone vapours under low pressure (200 mbar)leads to an x-like structure with low crystallinity, uniform coverage and no preferential ordering. Longer exposure in a saturated acetone vapour atmosphere results in an important phase transformation to the α form which leads to crystallites up to several micrometres large lying in the plane of the substrate and forming a discontinuous coverage. In the case of x powders treated in liquid acetone, a phase transformation to the α form of PcLi is observed. This x → α phasechange induced by the acetone vapour treatment is interpreted in terms of crystal size effects and compared with the phase transformation observed in thin films deposited at various substrate temperatures in the range 25–225 °C.

The polymorphism of lithium phthalocyanine (PcLi) was studied for powders as well as for thin films deposited on glass substrate. Powders were yielded by two electrochemical synthesis routes. While the powders obtained by electrochemical oxidation of PcLi2 in acetonitrile and acetone lead to an x structure, the synthesis route proposed by M.A. Petit leads apparently to the α form of PcLi. Their very different sensitivity to oxygen, observed using electron spin resonance (ESR) spectroscopy, is explained by means of the crystallographic structure of both polymorphi. Thin films were prepared by vacuum deposition and phase transitions as well as preferential ordering of the crystallites are studied as a function of increasing substrate temperature. While films deposited at low substrate temperatures, below 150°C, lead to an x-like form with the molecular columns lying in the plane of the substrate, the higher substrate temperatures favour the growth of the α form and a loss in preferential ordering. Accordingly, changes in the sensitivity to oxygen are followed as a function of substrate temperature.